Background Images - I'm unable to remove, reposition, scale or change
Its look like you are trying to add 2 images, 1 for the front view and 1 for the side view.

As per looking your screenshot and highlighted below #1 and #2. 1 is telling us that you are on the front view and #2 is telling that you are trying to adjust the side view of the picture. 

If you want to adjust size and stuff for the Front image, please use the settings under the front view as marked #3 below. or if you want to adjust the size of the side view you may use the side view, hit Numpad 3.